Fluidized bed treatment of EAF dust

ABSTRACT

The invention concerns a method of treating electric arc furnace (EAF) dust. In the method the dust is preheated and decontaminated under conditions which oxidize the magnetite content of the dust to hematite. Thereafter the preheated and decontaminated dust is introduced into a fluidized bed reactor in which hematite is reduced, by means of a hot reducing gas generated by reforming natural gas in a non-catalytic plasma-arc heating process, to yield an iron-rich material suitable for recycling to the EAF, and a high grade zinc oxide product.

BACKGROUND TO THE INVENTION

This invention relates to the treatment of electric arc furnace (EAF)dust.

In general, for every ton of liquid steel that is produced in minimills, i.e. steel mills with a capacity of the order of one million tonsof liquid steel per annum, approximately 15 kg of EAF dust is produced.It is estimated that in excess of three million tons of EAF dust isproduced worldwide each year by carbon steel manufacturers. The EAF dustwhich is produced is designated by certain authorities, such as theEnviromnmental Protection Agency in the USA, as a hazardous wastematerial inter alia because of high zinc content. As a result it isnecessary to treat the dust rather than merely dump it,

Traditionally, EAF dust is transported to a central treatment facility.One traditional treatment technique is the Waelz Kiln process operatedby Horsehead Industries in the USA. However, treatment of the EAF dustin this way presents several serious problems, including the fact thatthe actual cost of EAF dust treatment is extremely high. This is atleast partly because of the high capital cost of treatment facilitiesoperating the process. The high capital cost means that facilitiesoperating the process tend to have high capacities and furthermore thata single, central facility is usually provided to serve steel producersspread out over a large area. Transportation of the dust from the steelmill, often over large distances, contributes to the overall high costof dust treatment using the process. Added to this, the hazardous natureof the dust, together with its small particle size, typically less thanone micron, means that the actual transportation thereof from the steelmill to a treatment facility requires special handling measures, againincreasing the overall costs of treatment.

A typical EAF dust may include the following constituents by weight:

ZnO 27% PbO 2% Fe₂O₃ 44% C 6% Halides 2 to 4% Cd 220 ppm

A zinc oxide content of this magnitude can be attributable to thesmelting of large amounts of galvanised scrap in the steel mill.

An objective of the present invention is to provide a method whereby EAFdust can be treated on site in an economical manner. Given the high Znand Fe content of the typical EAF dust, another objective of the presentinvention is to provide for economical recovery thereof.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided amethod of treating EAF dust, the method comprising the steps ofpreheating and decontaminating the dust under conditions which oxidisethe magnetite content of the dust to hematite, and thereafterintroducing the preheated and decontaminated dust into a fluidised bedreactor in which hematite is reduced, by means of a hot reducing gasgenerated by reforming natural gas in a non-catalytic plasma-arc heatingprocess, to produce an iron-rich material suitable for recycling to theEAF.

In addition, the method may comprise the step of recovering ZnO fume(i.e. powder) from the fluidised bed reactor, typically in abag-house/filter system located downstream of the fluidised bed reactor.

Typically, the reducing fluidised bed reactor is operated at atemperature of the order of 800° to 1000° C., preferably 850° to 1000°C., most preferably about 950° C. The hot reducing gas typicallycomprises a mixture of H₂ and CO. In the preferred method the EAF dustis initially preheated in an oxidising fluidised bed reactor, typicallyto a temperature of the order of 1000° C., by heat derived fromoff-gases produced in the reducing fluidised bed reactor.

These gases may, after passing through a water scrubber, be heated inthe presence of air before being introduced into the oxidising fluidisedbed reactor.

Contaminants, such as halid es and Cd, may be removed from the oxidisingfluidised bed reactor through a water scrubber whereafter they may bedisposed of.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described in more detail, by way of exampleonly, with reference to the accompanying flow diagram which illustratesthe steps of an EAF dust treatment method according to the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

In the first step in the method represented by the accompanying flowdiagram, EAF dust produced from an EAF steel plant is introduced into afirst fluidised bed reactor 10, as indicated by the numeral 12. In thereactor 10, the EAF dust is preheated by off-gases produced in thesecond fluidised bed reactor, as described below The off-gases arepassed through a burner 14 supplied with air 16 before being introducedinto the fluidised bed reactor 10 at a temperature sufficient to preheatthe EAF dust to a temperature of the order of 1000° C.

In the oxidising and elevated temperature conditions prevailing in thereactor 10, the magnetite content of the EAF dust is oxidised tohematite by the following reaction:

2Fe₃O₄+{fraction (1/2 )}O₂→3Fe₂O₃

Gases produced in the reactor 10 are withdrawn through a water scrubber18 in which important contaminants typically present in the EAF dust,such as halides and Cd, are removed. The balance of the gas is exhaustedto atmosphere through a stack as indicated by the arrow 20.

The preheated and decontaminated dust is then introduced into a secondfluidised bed reactor 22 as indicated by the arrow 24. The secondfluidised bed reactor is supplied with hot reducing gas along the line26. The hot reducing gas, which is produced by reforming natural gas 28,together with steam, in an independently powered, non-catalyticplasma-arc reformer 30, comprises a mixture of H₂ and CO, typicallyapproximately 75% H₂ and 25% CO. The non-catalytic plasma-arc reformer30 is operated at a temperature sufficient for the production of a hotreducing gas, typically having a temperature of about 950° C., butgenerally in the range 800° to 1000° C. and preferably in the range 850°to 1000° C.

In practice, the non-catalytic plasma-arc reforming process which takesplace in the reformer 30 makes use of a plasma heating torch,incorporating an anode and a cathode, which is connected to the reactionchamber of the reformer by a sliding valve. The natural gas (CH₄) isreformed stoichiometrically together with steam at very hightemperatures generated by the torch, typically of the order of 15000°C., in the absence of a catalyst, according to the following reaction:

CH₄+H₂O→CO+3H₂

In the reactor 22, the hematite is reduced to an iron-rich product 32,i.e. direct reduced iron (DRI), according to the following reaction:

2Fe₂O₃+3H₂+3CO→4Fe+3H₂O+3CO₂

The DRI is suitable for recycling to the EAF of the steel plant. Thusthe iron content of the EAF is usefully recovered.

The ZnO and PbO in the EAF dust is reduced by the H₂ and CO to metalbut, importantly, under the operating conditions prevailing in thereactor 22 both the Zn and Pb are in the vapour phase and are carried bythe fluidising gas downstream to a cooler 34. At about 800° C. andbelow, the Zn back-reacts with the oxygen present to form ZnO. It isconsidered undesirable for the latter reaction to take place in thereactor 22 because this would lead to the possibility of mixing betweenthe iron rich material and ZnO fume which could in turn result in lowrecoveries of ZnO fume and in the iron being contaminated with ZnO.

Cooling in the cooler 34 to temperatures of 800° and below thus resultsin the oxidation of Zn and Pb to form a ZnO-rich fume 38 which isrecovered in a bag-house and filter system 36. The fume 38 is a highgrade, zinc oxide-rich powder with a low halide content. This fume issuitable for resale to primary electrolytic zinc producers. Theoff-gases produced by the reactor 22 are scrubbed in a water scrubber 40and thereafter heated in the burner 14, as described above, for thepurposes of preheating and decontaminating the raw EAF dust underoxidising conditions in the reactor 10.

It will be noted that the raw EAF dust is effectively converted to yieldtwo useful products, namely a marketable ZnO-rich fume and an iron richmaterial which can used in the EAF steel plant, with little or no waste.Apart from the fact that the EAF dust is converted to yield usefulproducts there is the added advantage that the process of the inventioncan be carried out economically in situ at the steel plant itself.

EXAMPLE

The following example describes a laboratory scale test carried out atProcedyne Corporation of New Jersey, USA using a 6 inch diameter Inconel601 fluidised bed reactor. The EAF dust used as feed material wassupplied by North Star Steel of Michigan, USA. The product samples wereanalysed by Mintek of South Africa. The general objective of the testwas to convert EAF dust into a high grade zinc oxide product and ironrich material without substantial waste products being generated.

A specific objective of the test was to produce a high grade,dehalogenated zinc oxide product with a zinc oxide content of greaterthan 80% by weight and an iron rich material metallised to a level of atleast 90% by weight.

The reducing fluidised bed reactor produced material having thefollowing properties:

%ZnO (by weight)—88

%Fe₂O₃ (by weight)—7,8

%PbO (by weight)—0,05

It will be seen that the specific objective of a zinc oxide content ofgreater than 80% was successfully achieved. The product of the processcan accordingly be regarded as a super high grade zinc oxide product.

Metallisation, i.e. production of metallic iron, levels of 98% were alsoachieved in the test. In addition dehalogenation, i.e. chlorideextraction, levels of greater than 90% were achieved.

The test results indicate clearly that EAF dust can successfully beconverted to yield a high grade zinc oxide product suitable for resaleto, for instance, zinc smelters for the production of zinc metal, and aniron rich product suitable for recycling to the EAF itself or to othersteel producers.

It is believed that the success of the method of the invention islargely attributable to the use of the hot reducing gas, generated byreforming natural gas in a non-catalytic plasma-arc reformer, in thereducing fluidised bed reactor.

What is claimed is:
 1. A method of treating EAF dust, the methodcomprising the steps of preheating the dust under conditions to removecontaminates including halides therefrom and to oxidise magnetitecontained therein to hematite, and thereafter introducing the preheateddust from which contaminants have been removed into a fluidised bedreactor in which said hematite is reduced, by means of a hot reducinggas generated by reforming natural gas in a non-catalytic plasma-archeating process, to yield an iron-rich material suitable for recyclingto the EAF.
 2. The method of claim 1 and comprising the step ofrecovering ZnO-rich fume from the product of the fluidised bed reactor.3. The method of claim 1 or claim 2 wherein the reducing fluidised bedreactor is operated at a temperature in the range 800° to 1000° C. 4.The method of claim 3 wherein the reducing fluidised bed reactor isoperated at a temperature in the range 850° to 1000° C.
 5. The method ofclaim 4 wherein the reducing fluidised bed reactor is operated at atemperature of about 950° C.
 6. The method of claim 1 wherein the hotreducing gas comprises a mixture of H₂ and CO.
 7. The method of claim 1wherein the EAF dust is initially preheated in an oxidising fluidisedbed reactor, by heat derived from off-gases produced in the reducingfluidised bed reactor.
 8. The method of claim 7 wherein the EAF dust ispreheated to a temperature of the order of 1000° C.
 9. The method ofclaim 8 wherein the off-gases, after passing through a water scrubber,are heated in the presence of air before being introduced into theoxidising fluidised bed reactor.
 10. The method of claim 1 whereinhalides and Cd comprise contaminants in the in said EAF which areremoved from the oxidising fluidised bed reactor through a waterscrubber and are thereafter disposed of.